Different manners of producing nuclear fuel pellets are known by a person skilled in the art. It is common to make the nuclear fuel pellet from a nuclear fuel material in powder form. The nuclear fuel material may for example be UO2, where U is enriched with regard to 235U. The powder material may also include other additives, such as U3O8 and binder material. The powder is pressed in order to form a so-called green pellet. The concept “green pellet” in this technical field means the pressed pellet before it is sintered. The green pellet is thus thereafter sintered in a furnace. The sintered pellets are thereafter ground in order to obtain the correct diameter and surface finish.
It is also known to include some additives in the powder in order to increase the grain size in the sintered pellet. For example WO 00/49621 A1 gives some examples of such additives and describes how the nuclear fuel pellet may be produced.
DE 3235944 A1 describes that a solution of hydrogen peroxide is added to the uranium dioxide powder in order to increase the grain size.
WO 2005/041208 A2 describes that a porous uranium dioxide arrangement is in-filtrated with a precursor liquid in the form of allylhydridopolycarbosilane in order to enhance the thermal conductivity in the nuclear fuel.
As a background of the present invention a further phenomenon should be mentioned. This phenomenon is a structure in the used nuclear fuel pellets called high burn-up structure (HBS) or rim structure. When the nuclear fuel has been used for a longer time in a nuclear reactor (i.e. a high burn-up) a new restructured configuration appears at the outer thin region of the fuel pellet. This phenomenon is described for example in the article “The high burn-up structure in nuclear fuel” by V. V. Rondinella et al. in Materials Today, December 2010, Volume 13, No. 12, pages 24-32. The HBS means that the grains in the outer region of the nuclear fuel pellet subdivide into very small grains. The outer region in which the HBS appears may for example be less than 100 μm thick.